In electrographic printing and copying, liquid toner compositions are frequently applied to an electrostatic latent image formed on a dielectric surface in order to develop the image. The dielectric surface may be a coating on a sheet or web of paper to which toner is applied. Alternatively, the dielectric surface may be the charge retentive surface of a drum, belt or the like from which toner applied thereto is transferred to a sheet or web of plain paper. The electrostatic latent image may be established through electrostatic induction by a charged writing head, by ion projection, or through photoconduction, as in photographic copiers or laser printers. The toner is a mixture having pigments or dyestuffs combined with a plastic or resinous binder, hereafter called "solid pigment particles" or "colorant", with very small amounts of added charge control agents, both dispersed in a large volume of liquid dispersant, primarily solvent. One commonly used solvent is an isoparaffinic hydrocarbon available under the trade name Isopar, manufactured by Exxon Corporation.
Whether the image is formed directly on a sheet of paper or is transferred to a sheet of paper after formation on a dielectric surface, the toned image surface will carry not only the solid pigment particles but also some amount of liquid dispersant. In the case of a transferred image, at least a portion of this remaining liquid dispersant will transfer to the paper sheet along with the image. After exit of the sheet of paper from the printer or copier, the liquid dispersant vaporizes. The emission of hydrocarbon vapor into the atmosphere of an office or workplace has been perceived as a matter for concern. A machine which continually puts out wet sheets may cause a build-up of hydrocarbon vapor above the prescribed safety limits and is likely to cause irritation to those persons in the area around the machine. This has become popularly known as the "sick building" syndrome.
Therefore, one aim in the design of printers and copiers is to reduce to a minimum the emission of hydrocarbons into the ambient atmosphere. U.S. Pat. No. 4,693,206 issued to Day and assigned to the assignee of this application teaches a drying roller which comes into frictional contact with a dielectric sheet after development of a latent image on the electrostatic sheet. The drying roller removes a large percentage of the excess liquid dispersant to produce relatively dry paper.
The use of airflow as an alternative to a drying roller is known. U.S. Pat. No. 4,198,923 to Blumenthal discloses a toner clean-off head which employs a high vacuum to insure proper engagement between the head and a sheet of paper. The sheet of paper is drawn over the curved clean-off head for removal of the excess toner by suction. Conversely, U.S. Pat. No. 4,383,019 to Simm teaches fixing an air jet member at an angle relative to an image carrier-band. The use of drying rollers, air suction, and the air jet member of Simm all provide a relatively dry sheet of paper, but it is recognized in the art that improvements may be made in drying apparatus. Moreover, air drying is often inadequate in reducing the emission of hydrocarbons, since the vapor-laden air carries the hydrocarbons into the ambient atmosphere. The efficiency of a drying apparatus is partly a function of the distance between the apparatus and the surface to be dried. In determining the position of the jets of Simm, the eccentricities of the surface to be dried must be considered thereby limiting the minimum distance from which the air jet can be projected.
It is an object of the present invention to improve upon the drying apparatus and to significantly reduce the emission of hydrocarbons from a printer or copier.